The present invention relates to a system for locking turbine blades on a turbine rotor wheel, as well as methods for assembling and disassembling the turbine blades and the turbine rotor wheel.
Known techniques for attaching blades to a turbine wheel include forming a circumferentially extending groove around the periphery of the turbine wheel, the groove having a generally dovetail shaped cross-section, and forming each of the turbine blades with an enlarged foot portion, having a cross-sectional shape similar to that of the circumferential groove. The interengagement of the enlarged foot portion on the blades with the groove formed in the turbine wheel serves to radially retain the turbine blades on the turbine wheel.
This system has proven very effective in accomplishing radial retention of the blades, but has created problems regarding the assembly and disassembly of the structures. As a rule, the axial dimension (the dimension measured in a direction generally parallel to the axis of rotation of the turbine wheel) of the enlarged foot portions formed on the turbine blades exceed that of the opening of the circumferential groove. Therefore, in order to insert the foot portions of the blades into the circumferential groove, at least one "window" must be provided on the rim of the turbine wheel. The axial dimension of this "window" or enlarged opening is sufficient to allow the radial insertion of the enlarged foot portions into the groove. Once inserted, the turbine blades may be circumferentially displaced away from the window such that the foot portions are radially retained in the circumferential groove.
In order to avoid the formation of such a "window" it is known to form the enlarged foot portion with a narrow circumferential dimension such that they may be inserted into the groove opening by turning them sideways. Once the enlarged foot portion passes into the circumferential groove, the blades are then rotated 90.degree. to engage the enlarged foot portions and the circumferential groove.
As is well known in the art, the turbine blades may be provided with platforms which, when the blades are assembled on the turbine wheel, delineate one side of the gas passage over the turbine wheel. The individual platforms form a substantially continuous ring when the blades are installed on the turbine wheel to define the wall of the gas passage. It is desirable to maintain this wall as a continuous annular formation to avoid any disturbances in the passage of the gases over the turbine wheel, such disturbances serving to reduce the efficiency of the turbine assembly. The platforms of the turbine blades which are installed into the turbine wheel by rotating 90.degree. must have at least opposite corners thereof rounded or otherwise removed to facilitate the rotation of the turbine blade once it is placed into the turbine wheel groove. Quite obviously, this will prevent the formation of the continuous annular structure desired to maximize turbine efficiency.
By using an enlarged opening or "window", the rounding or removal of corners of the turbine blade platforms may be eliminated. However, quite obviously, some means must be utilized to lock the turbine blades in position on the turbine wheel such that they cannot be inadvertantly removed through the "window". The most rudimentary form of such locking mechanism involve bolts or nuts extending through the turbine blade platforms. These have proven totally unsatisfactory, since, not only do they create gas flow disturbances, but the bolts or nuts may loosen during turbine operation and cause catastrophic damage to the turbine.
Other known system utilize internal locking devices which are disposed below the annular ring formed by the turbine blade platforms and are, thus, out of the gas flow stream. However, means must be provided for attaching and removing such locking devices and, typically, these means necessitate the formation of an opening or orifice through one or more of the platforms. These openings also cause disturbances in the gas flow, thereby reducing the efficiency of the turbine.
French Pat. No. 1,541,373 describes a locking device that is located between two blade roots and wherein the blade platforms form apertures to allow insertion and removal of the locking device. Not only does the platform aperture form a disturbance in the boundary layer flow and degrade the compressor operation, such disturbances may produce vibrations that may unlatch the locking means and cause the blades to slip in the mounting groove.
A similar, but more complex, locking apparatus is disclosed in U.S. Pat. No. 4,462,756. This locking means still requires apertures to be formed in the blade platform, resulting in the aforementioned deficiencies.